Photo-sharing startup Instagram has been on something of a hiring spree in August 2011, boosting its headcount by 50% to cope with growth that has seen its app downloaded by more than eight million iPhone owners since its launch in October 2010.

For many startups, such a recruitment drive would involve moving into bigger offices. In Instagram's case, it was more a case of finding a couple of extra chairs and desks. Yes, the most startling thing about the San Francisco-based company is that it currently only has six employees.

"We've been blown away by the response from around the world since we launched," says CEO Kevin Systrom. "We never expected to see that, and it's been really humbling. We started as just two guys in an incubator working on a project, and now we're six people, and have been handed the opportunity to do something really big in the world. We have a responsibility to see this through, and it's an amazing ride."

Instagram is far from the only mobile app offering people a way to share their photos on Twitter, Facebook, Flickr and other social services, nor is it the only one to include filters to apply visual effects to the shots before they are shared. It is certainly the one that has done most to break out beyond an early adopter audience, though.

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Systrom thinks that Instagram has done a good job of providing filters for users to "modify and improve" their photos, but accepts there are many free and paid apps offering similar effects. So why did Instagram catch on?

"There's this large network underneath where people can connect and follow inspiring photographers and friends around the world," says Systrom.

"That's what a lot of other apps don't provide, but we've combined it with the effects. We tried to lower the bar for producing awesome stuff, so people can walk around with their iPhones and produce amazing pictures. The network really democratises attention: everyone from celebrities to a random guy in Japan taking pictures of his dog every day can get many thousands of followers. Taking images is the great equaliser."

The celebrities are increasing in number, including Snoop Dogg, Jamie Oliver and Justin Bieber – whose presence is often seen as the ultimate seal of pop-star approval for a buzzy social service (see also: Twitter, Tumblr). Brands have also been attracted to Instagram since its launch, with the likes of Starbucks, National Geographic, Playboy, Red Bull and Gucci all posting images on the service.

This particular group of new users poses an interesting challenge for Instagram: should it start developing some new features with celebrities and brands in mind?

In August, Fred Wilson of venture capital firm Union Square Ventures published a blog post titled "Users First, Brands Second", in which he wrote approvingly of companies including Twitter, Facebook, Foursquare and Tumblr. "The brands are all over these services now. But for the most part, these services didn't do much to bring them. The engaged users did," wrote Wilson. Instagram fits neatly into that list too.

"We want to make sure all the features we build, even when they make sense for celebrities or brands, are big-impact changes that affect everyone," says Systrom. "It's important not to get distracted. Some people have maybe over-stated the value of getting celebrities and brands from the start. If you focus on producing a great experience for anyone, that's how you get big."

"Justin Bieber has certainly added to the velocity, but the press probably played it up a little bit," laughs Systrom. "We get almost one signup every second, and it's been fine: even when celebrities have signed up, we've not had any stress on the system."

With "well over eight million" downloads now, what next for Instagram? Systrom says that the biggest challenge for the company is scaling, with a lot of the work going on below the surface to ensure the service continues running smoothly as it grows. As he says, this work isn't visible to users, but if the company doesn't do it, it will be extremely visible: "The app won't work!"

Systrom outlines three other key tasks for Instagram in the coming months, starting with revamping the core camera feature. "We always wanted to be the social camera, but the photo-taking part of the app has not changed in eight to nine months," he says.

"It's way overdue for some really core improvements. The second thing is the web: we realise that people want a native Instagram experience on the web, and we need to do that sooner rather than later."

And third? Android. Systrom says that every day, Instagram gets a barrage of requests from people for an Android version of its app. The good news is that it's on the horizon.

"It's hugely important to us, but we're only six people," he says. "Android is a major priority for us, but first we have to build the team, and find the best people in the world to work on these projects." The company is currently hiring engineers and designers to help it move more quickly on all three of these aims.

There is also a fourth titbit: new filters. "We have a really big release coming out soon that really changes the way you look at filters," say Systrom, while warning that Instagram intends to continue keeping the experience simple and straightforward, rather than bombarding users with hundreds of possible visual effects.

It's impossible to describe Instagram in 2011 without referring to the 900lb gorilla looming in the distance: Facebook. The social network is expected to add Instagram-style filters to its own mobile apps in the near future, or even launch a separate photo-sharing app. Cue talk of Facebook being a potential Instagram-killer.

"I understand why someone like Facebook, where photos are core to their experience, would have things like filters," says Systrom. "But the core experience is so different. We use asymmetric follows like Twitter, which is a really big differentiating point. My friends network doesn't necessarily take the best photos. The major reason why Instagram works is that you can follow anyone out there and start following their photos immediately."

He thinks adding filters will make sense for Facebook, but maintains that this will not be direct competition for Instagram.

"It's more of a press-cycle thing: it's been sensationalised beyond the reality of what the situation is," says Systrom, while also pointing to the fact that Instagram enables photos to be shared on a range of social networks, which is unlikely to be the case for Facebook's apps.

One good omen: in 2010, the Facebook Places location feature was talked of as a potential Foursquare-killer, yet it is now being phased out by the social network in favour of wider location features, while Foursquare continues to grow. Systrom will be hoping that any threat posed by Facebook to his own company turns out to be a similarly damp squib.